Somali pirates attacked a French naval vessel on Wednesday, mistaking it for a commercial cargo ship. (Five pirates were captured.) A similarly ill-advised assault was launched on a different French naval ship several months ago, and back in September, pirates inadvertently went after a U.S. naval vessel. Is it really that hard to distinguish a military ship from a commercial freighter?
Sometimes. Warships would be easy to identify on account of their distinctive shape and enormous radar systems. But the navies of most countries also deploy cargo ships, refueling tankers, personnel transports, repair ships, rescue tugs, and self-propelled floating cranes, to name just a few. These are similar in size and silhouette to civilian ships that serve the same function. There are a couple of distinguishing features. A European ship, for example, will have a pennant number painted somewhere on its hull. Many support ships also carry 3- or 5-inch guns on the bow or stern. These giveaways may be hard to spot if it's dark or if you're too far away. In that case, there is almost no way of knowing whether a cargo ship belongs to a navy. The U.S. Navy has taken advantage of the confusion, converting portions of cargo ships like the USNS Lewis and Clark * to floating prisons for discombobulated buccaneers.
An unwitting pirate might mistake an auxiliary naval ship for an especially attractive target. Any vessel large enough to carry heavy cargo—like tanks—would seem likely to contain a lot of booty. The French replenishing oiler attacked on Wednesday is 515.7 feet long by 69.5 feet wide and can deliver 9,250 tons of fuel, 250 tons of water, and 190 tons of food to patrolling warships. It also has a sizable deck for helicopters, which might be mistaken for a wide expanse of pallets filled with valuable commercial goods.
The French ship also serves as a command vessel, so it comes equipped with a fuller range of armaments than many refueling ships. The ship was stocked with a small number of surface-to-air missiles, which are housed and launched from inside the ship's body. (Twenty years ago, the missiles would have been mounted conspicuously on the deck.) Its set of 20- and 40-millimeter anti-aircraft guns might have been more obvious, as these can protrude up to 10 feet. Such weapons are typically mounted low to the hull, however, and might be missed in the darkness.
It would have been much easier for the pirates to distinguish a true warship from a private vessel. Warships' hulls have lots of curves to make them faster on the water and harder to pick up on radar. Even at night, their silhouettes are very distinctive. Confusing a destroyer with a commercial cargo ship would be a bit like mistaking a sports car for a tractor trailer. Warships also tend to carry significantly larger radar equipment than other vessels. When pirates do spot an attack ship, they dump their weapons overboard to eliminate evidence of piracy.
Even when the pirates manage to avoid ships owned by a national navy, they have to worry about private charters. The U.S. military often rents out civilian vessels to ferry weapons from friendly ports to war zones. While these ships don't carry offensive missiles, they are staffed by a heavily armed security contingent from the U.S. Navy.
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Explainer thanks Eric Wertheim, author of the Naval Institute Guide to Combat Fleets of the World: Their Ships, Aircraft, and Systems (15th edition).
Correction, Oct. 9, 2009: The original version of this article mistakenly referred to the USNS Lewis and Clark as the USS Lewis and Clark. (Return to the corrected sentence.)